The present invention relates generally to the manufacture of plastic film bags and, in a preferred embodiment thereof, more particularly provides apparatus and methods for attaching to the bags plastic film tie elements which, as disclosed in my copending U.S. application Ser. No. 117,209, may be tied around the open tops of the bags to tightly close them.
In the manufacture of plastic film bags it is common practice to form them, by continuously extruding plastic film in tubular form, flattening the film tube to form a double layer "web", forming lateral weld lines and perforation lines across the web to define the individual bags which may be subsequently separated from one another at the perforation lines, and then laterally folding the web prior to packaging of the bags. The laterally folded film web is then typically delivered to a packaging station spaced apart from the bag forming station, at a linear receiving speed identical to the linear output speed of the bag forming station, where it is rolled or folded for packaging.
For the purpose of attaching accessories to, forming logos or heat seals on, or otherwise modifying the individual bags prior to their receipt at the packaging station, it is desirable to momentarily stop the web at each bag during the performance of a particular modification operation thereon--for example, the attachment of a plastic film tie element disclosed in U.S. application Ser. No. 117,209 incorporated by reference herein.
There are presently two methods for effecting this necessary momentary stoppage of the web as each individual bag passes the modification station--neither of which is wholly satisfactory. First, both the bag forming station output feed portion and the packaging station input drive may be synchronously operated in a start-stop fashion to incrementally advance and then stop the entire folded film web section extending between these two operating stations. While this is a quite logical approach, it significantly slows the overall bag production rate --a rate which must be kept as high as possible for profitability purposes.
Second, a rather complex, high mass, shiftable multiroller structure may be utilized to engage and intermittently stop a portion of the folded plastic film web between the bag forming station and the winder without slowing or interrupting the output and input web travel at these portions of the overall bag forming apparatus. However, this high mass roller structure must be very rapidly shifted back and forth to stop each individual bag received thereby during the high speed bag forming process. Because of the rapidity with which the multi-roller structure must be intermittently shifted back and forth, very high shift forces result, requiring substantial power and precision control. If the multi-roller structure is not precisely designed and adjusted, these high shift forces can easily tear the travelling film web at one of its perforation lines, creating significant down time and waste in the bag manufacturing process.
In view of the foregoing it can be seen that improved apparatus and methods for momentarily stopping each individual bag in the film web, during its movement between the bag forming station and the winder, are needed. It is accordingly an object of the present invention to provide such improved apparatus and methods.